Communication system



Jung 13, 1939. R. H. CAMPBELL 2,152,372

OHIUNICATION SYSTEM Filed NOV. l, 1957 PatenteciJune 13, 1939 UNITEDSTATES PATENT CFFICE COMMUNICATION SYSTEM poration of WisconsinApplication November 1,

27 Claims.

i The present invention relates to intercommunication systems andparticularly to telephonic communication systems comprising a pluralityof remotely located stations, each supplied with suitable voicereceiving and transmitting means, and means for amplifying fluctuationsin electrical current created in the transmitting means.

One of the primary objects of the present invention is the provision ofa communication system comprising a number of remotely located stations,each including amplifying means and a pair of loudspeaker units of thepermanent magnet type, of which one is used as a transmitter and theother as a receiver, in which feed back between the receiving andtransmitting means is substantially eliminated.`

A more specific object ofthe invention is to decrease the feed back whenboth loudspeaker o units at each station are used by reducing the outputof the receiver so that less sound `energy is transmitted from thelatter to the transmitter upon establishment of two-way communication. Afurther object of the invention is to reduce the feed back by placingresistances in series with the receiver loudspeaker units uponestablishment of two-way communication, and to accomplish this withoutdisturbing the impedance match between the output transformers of ...mthe ampliers and the receiver circuits which include the aforementionedreceiver loudspeaker units and resistances.

A further object of the invention is to provide a multiple stationintercommunicating system of the' type described above with controlmeans having a normal position in which the B supply to the amplifier iscut off and the output terminals of the latter are short-circuited toprevent ampliiication of any extraneous electrical n) disturbances thatmight be impressed across its input terminals and at the same time avoidundesired clicks.

A further object of the invention is to provide an intercommunicationsystem that can be se- 45 lectively operated to obtain full two-waycommunication at reduced volume without manipulation of switches afterthe selection is made or operated to obtain similar results at onestation i" at full volume, and a system in which this selection can madein a -simple manner.

Other objects and advantages of the present invention will becomeapparent from the ensuing description, in the course of which referenceis 55 had to the accompanying drawing, the single '1931, serial No.172,103

(cl. 17e- 1) figure of which represents schematically one embodiment ofthe invention.

Referring now to the figure, reference numerals I and 2 indicate twostations of a multiple station intercommunication system. These stationsare located remote from each other and lfrom the remaining stations ofthe system which have been omitted from the figure for the purpose ofsimplicity and brevity, but which are provided with apparatus identicalto that found at the illustrated stations.

The stations are provided with amplifiers II and I2, voice receivingmeans I3 and I4, and voice transmitting means I5 and I6, respectively.These are identical in construction and preferably are loudspeaker unitsof the permanent magnet type now commonly utilized in systems of thetype to which the present invention pertains.

The communication circuits and the supply of 2( power at the twostations are controlled by combined power and communication controlswitches, I1 and I8. These switches are multi-pole, multipositionswitches constructed and arranged in the various circuits in such mannerthat they occupy a normal position in which the stations are conditionedfor reception of a call and the amplifiers cle-energized and areoperable to a` plurality of displaced positions. They are operable to afirst displaced position in which the stations are conditioned fortransmission, and from which position they arereturned to their normalposition by suitable biasing means, to a second displaced position inwhich the stations are conditioned for transmission at full volume andreception at reduced volume, and to a third position in which thestations are conditioned for both transmission and reception at fullvolume. When switches at two stations are operated into their seconddisplaced positions, in which they are adapted to remain until manuallyoperated to another position, two-way communication may be carried onbetween stations without further manipulation of switches. When theswitch at one station is operated into its third 45 displaced positionthat station is conditioned for reception and transmission at fullvolume, but, in order to prevent feed back it is necessary for theswitch at another station to be operated in such manner as to renderineffective either the receiver or transmitter, depending upon thedirection of communication. 'I'he switch construction and circuitarrangement whereby these various conditioning operations are effect- 55ed and the advantages thereof are described shortly.

To enable operators at each station to communicate with any or all ofthe remaining stations to the exclusion of unselected stations, thereare provided a number` of independent inter-station conductors equal tothe number of stations, a single common conductor, and a stationselector. In the figure, where inter-station conductors 2| to 24,inclusive, have been illustrated, conductors 2| and 22 are theinter-station conductors associated with stations I and 2, respectively,while the others are associated with stations not shown. The commonconductor, indicated by reference numeral 25, is grounded and extends toall stations as do the other interstation conductors. The selector meansare indicated by reference numerals 25 and 2'I and the former, which islocated at station I, consists of a plurality of switch blades 28 to 30,inclusive. These blades are adapted to be actuated into engagement withcontacts connected by conductors 32 to 34, inclusive, to inter-stationconductors 22 to 24, respectively. The blades are all connected to acommon conductor 36 leading to the power and communication controlswitch I1, which is adapted selectively to connect the conductor totheoutput of the amplifier in a manner hereinafter described.

The amplifier II at station I may be of any wellknown type comprisingone or more stages of audio amplification and is preferably providedwith transformers having input and output coils 31 and 38, respectively,each of which has one terminal connected to the common conductor 25. Itshould be understood that other forms of input and output connectionsmay be utilized in place of the coils illustrated.

The power and communication control switch consists of four blades 4|to.44, inclusive, all mechanically interconnected for simultaneousoperation and biased from its first displaced position to its normalposition, corresponding to that indicated, by spring 45. The switch isoperable to two other displaced positions in either of which it isadapted to remain until manually operated therefrom.

'I'he amplifier is supplied with power from a suitable source (notshown) and according to the present invention the cathodes of thevarious amplifier tubes are continuously heated to condition theamplifier for instantaneous operation while the operativeness of theamplifier is controlled by controlling the B" supply. The B" supply iscontrolled by switch blade 4| which is normally in circuit openingposition, thereby cutting off the B supply to the amplifier andeffecting not only a saving in power but also preventing amplificationof undesired local electrical disturbances. In each of the threedisplaced positions, of which the first is below the normal one and theother two above, the B supply circuit is completed through conductors 4land 48.

The voice coil of transmitter I is permanently connected to the input ofthe amplifier while the connection of the Voice coil of receiver I3 tothe inter-station conductor 2| associated with station I is controlledby switch blade 42. In the normal position of the blade the voice coilis connected' directly to the inter-station conductor 2| throughconductors 53 to 56, inclusive, so that station I may be called by anyof the other stations in the system. In the first dispmced position ofmade 42 the connection of the receiver to the inter-station conductor isbroken. This position is termed the talking position in which thestation is conditioned for transmission alone.

In systems comprising both receivers and transmitters at each of aplurality of stations, there is a likelihood that acoustic feed backwith its attendant howling will occur. This is particularly true wherethe receiver is of the loudspeaker type and amplifiers are utilized toactuate them. In order to decrease the possibility of this feed back,the present system is so arranged so that when two or more stations areconnected for full two-way communication, after one station hasresponded to a call from another, the receiver output volume isdecreased. This permits utilization of full volume on reception of callsand decreases acoustic feed back upon establishment of two-waycommunication. The reduction of volume is accomplished by interposing aresistance 58 in series with the voi'ce coil of receiver I3 when theswitch blade 42 is actuated to its second displaced position.

In some installations it is desirable to operate the system in suchmanner that full volume is obtainable at one station, and in the presentembodiment this may be accomplished by actuating the blade 42 to itsuppermost or third displaced position. In this position the receiver isconnected directly to the inter-station conductor 2| through conductor54, just as in the first displaced, or calling, position. However, toavoid feed back the communication control switch I8 must be operatedselectively to disconnect the receiver and transmitter to open the feedback circuit at station 2.

The switch blade 43 is provided for the purpose of maintaining constantthe impedance of the receiving circuit when the resistance 58 is placedin series with the voice coil. Thus, when the switch is operated 4to itssecond displaced position the impedance is maintained constant byconnection of resistance 59 in parallel to the voice coil throughconductor 6|. Resistance 59 has a value dependent upon the resistancesof the loutput transformer of the amplifier at station 2, the resistanceof the voice coil of receiver I3, and the resistance 58. Assuming theresistances of the voice coil and output transformer to be 50 ohms each,and the resistance 58 to have a value of 25 ohms, then by applyingwell-known principles it may be determined that resistance 5S shouldhave a value of 50 ohms in order to maintain the receiving circuitresistance constant at 50 ohms. In practice of the invention, it is notnecessary that the impedance be maintained constant in the speciiicmanner described above; other arrangements may be utilized.

I'he fourth switch blade 44 is adapted in its normal position toshort-circuit the amplier output transformer through conductor 62 inorder to prevent clicks, amplification of undesired disturbances, and toprevent feed back that might otherwise result when the system isoperated in accordance with one of the contemplated modes of operation.The mode of operation referred to is that one where one station isconditioned for transmission and reception at full volume. Then, whenvthe other station changes over from transmitting to receiving there is alikelihood that feed back will occur for a moment because of the factthat cutting off the B supply on making such a change does notimmediately render inoperative the amplifier. By concurrentlyshort-circuiting the output transformer and cutl alecm ting gif the Bfsupply, even momentary feed back is eliminated. The switch bladeisadapted in all three of its displaced positions to connect the outputof the amplifier to conductor l and selector switch 2i.

The 'apparatus at station 2 differs from that at station I- only in themanner of connection of the selector switch 21 and receiver Il to theinter-stationconductors. The three blades il, II

and Il of selector switch 21 are adapted to complete a circuit tointer-station conductors 2l, 23

and `communication control switch Il. It is be- V.'iI)

lieved thatfurther description of the apparatus at station 2 isunnecessary because it corresponds exactly withthat found at station I.

With the power supply conductors connected to suitable sources of powerat each station and the switches in the normal positions illustrated,the system is conditionedfor the calling, at full volume, of onexor morestations by an operator at any one of the stations. For instance, theoperator at station I can call station 2 by closing switch 28 andoperating control switch I1 to its lowermost position. In this position,as previously described, the output of amplifier II is connecteddirectly to inter-station conductor 22 through switch blades Il and 28,while switch blade Il closes a circuit from conductor 41 to conductor I8to connect the B supply to the amplifier. Switches 42 and 42 are incircuit opening position, thereby disconnecting receiver i 3 at stationI from the system. Since transmitter I5 is permanently connected to theinput of the amplifier, speech is transmitted from station I to receiveri4 at station 2, which is, when switch I8 is in its normal position,connected directly to its associated inter-station conductor 22.Receiver I4 is operated at full volume and persons anywhere in thevicinity thereof the apprised of the fact that an operator at some otherstation is calling.

If the operator at station 2 desires to respond to the call, he closesselector switch 68 connecting the output of the amplier I2 tointer-station conductor 2i, the inter-station conductor associated withstation I, the operator at station I having in the meantime released hisswitch, which returns to its indicated position, If the operators thendesire to establish two-way communication, the communication controlswitches are actuated into their second displaced positions, in which itmay be remembered, they are adapted to remain.' In these positions,resistance 58 at station I and a similar resistance at station 2 areplaced in series with the receiver voice coils to reduce the volume andprevent feed back, without disturbing the impedance match between theassociatedoutput transformer coil and receiving circuit in a manner thathas been previously described. After the conversation has beencompleted, the operators simply return the communication controlswitches to their normal positions and condition the stations forfurther calls.

If the operator at one station, for instance station 2, desires tooperate at full volume instead of reduced volume because of a high noiselevel at the station, as described` in the previous paragraph, he can doso by operating control switch 21 into its uppermost or third displacedposition.

This operation results in the connection of receiver Il directly tointer-station conductor 22 and the transmitter to inter-stationconductor 2l through thev amplifier. `In, orderto eifect'twowaycommunication, it is now necessary for the operator at station I tooperate control switch from its iowermost or talk position to transmitto station 2 and to release the switch to receive from station 2.`Transmission from station I is accomplished through ampliner Il, theoutput of which is connected to inter-station conductor 22. Duringtransmission the receiver Il is disconnected from inter-stationconductor 2| at switch blade I2, so that the feed back circuit isbroken. Reception at station I is accomplished through ampliiier I2atstation 2 and during reception the "B" supply to amplier II is cut offand the output transformer of the latter shortclrcuited. As explainedpreviously, feed back is preventedA even though the ampliiier is notimmedlately rendered inoperative by cutting on the supply.

The operators at the various stations can initiate communication also byoperating the communication control switches to their second displaced'`positions. This mode of operation has the advantage that if the operatorat a called station responds by similar operation of the control switchthereat, the system is immediately conditioned for full two-waycommunication at reduced volume. i

The described system may be advantageously operated in another modewithout occurrence of feed back. According to this mode the operator atthe station desiring to talk has merely to operate his control switch toits lowermost position,

`and the operator at the other station permits the control switchthereat to remain in its normal position. When it is desired to'reversethe direction of communication, the positions of the switches at the twostations are reversed.

' The preceding portion Yof the description of operation has beendirected solely to communication between stations I and 2. However, ifthe operator at station'l desires to communicate with a third station,he would simply close switch 29 or 352, depending upon which of theremaining sta- 'tions he desires to communicate with. Thereafter, theoperation would be substantially the same as described above. It mightbe noted at this point, that when the operator atstatlon I iscommunicating with the operator at station 3,

the operator at station 2 cannot enter into the communication unless theoperators at stations I or 2 permit'it. This results from the fact thatthe receiver at station 2 is connected to the interstation conductor 22so that it is possible for the operator at station 2 to listen only whenthe selector switches at the other stations are closed to connect theoutput from the stations to the inter-station conductor 22.

The system can be conditioned for s. conference between a plurality ofstations, simply by closing all of the various selector switches,thereby to connect all the stations in parallel.

While only a single embodiment of the invention has been illustrated anddescribed, it should be understood that it is susceptible of other andfurther mcdications. For instance, certain features of the invention areadapted for use in sysshould be understood further, that the specific 4e embaimens u not intended es limit' the scope of invention in any way.r Y,

United States letters Patentis as follows:

l. The method of preventing feed back in a multiple stationintercommunication system in which each station is provided with aseparate voicereceiving and transmitting means adaptedtobeconnectedinthesystemsothatthetransmitting means at one station isconnected to the receiving means at another forl simultaneous twowaycommunication through an interstation circuit, and with ampliiler meansin circuit between system, the combination including, separate voicereceiving means and transmitting means at each station, 'switch means ateach station adapted to connect the reciving means thereat for receptionat high volume, said switch means being operable to a displaced positionto connect both the receiving and transmitting means in the system fortwo-way pommunication, and means for reducing the volume of thereceiving means adapted to be rendered effective when said switch meansis operated to its displaced position.

3. In an intercommunication system, the combination including, aplurality of stations, each having voice receiving and transmittingmeans adapted to be connected so that the transmitting means at onestation is operatively connected to the receiving means at another, aninterstation circuit for so interconnecting the. stations, ampliilermeans in circuit between associated transmitting and receiving meansadjusted to amplify the messages to a volume sufficiently high to createfeed back if the stations are connected for simultaneous two-waytransmission, volume decreasing means at each station, and meansincluding circuits and control devices at each station adapted tocondition each station for reception at high volume through theinterstation circuit and operable to render said volume decreasing meanseffective when the transmitting means at that station is operativelyconnected in circuit with the receiving means at another to completeconnections for simultaneous.twoway commimication.

4. `In a multiple station intercommunicationv system, the combinationincluding, separate electrical voice receiving means and transmittingmeans at each station, switch means adapted to connect said receivingmeans in the system for reception at high volume, said switch meansbeing operable to a displaced position to connect both the receiving andtransmitting means in the system for two-way communication, andresistance means adapted to be placed in circuit with said receivingmeans for reducing the volume of said receiver when said switch means isoperated into its displaced position.

bination includinga plurality 'of stations, each having receiving andtransmitting circuits, said receiving circuits having an impedancesubstantially equal to that of the transmitting circuits, voicereceiving means in said receiving circuits, impedance means adapted tobe placed in circuit with the voice receiving means to reduce the outputvolume thereof and to maintain substantially constant the impedance ofsaid receiving circuit, and communication control means adaptedsimultaneously to eifect interconnection of said receiving andtransmitting circuits and to place said impedance means in circuit withthe voice receiving means.

`6. In an intercommunicating system, the combination including aplurality of stations, separate voice receiving means and transmittingmeans at each station, amplifiers at each station having input andoutp'ut coils, said output coils having impedances matching those of thereceiving means. switch means adapted to connect said receiving means inthe system for reception at high volume, said switch means beingoperable toa displaced position to connect both the receiving andtransmitting means in the system for two-way communication, andresistance means adapted to be placed in circuit with said receivingmeans for reducing the volume of said receiver whensaid switch means isoperated into its displaced position, said resistance means being soproportioned-and arranged with-respect to the receiving means that thecombined impedance thereof remains substantially the same as that of thereceiving means alone. l 7. In an interfommunicating system, thecombination including a plurality of stations, separate voice receivingmeans and transmitting means at each station, amplifiers at each stationhaving input and output coils, said output coils having impedancesmatching those of the receiving means, switch means adapted in a rstiosition to connect said receiving means in the system for reception athigh volume, said switch means being operable to a displaced position toconnect both the receiving and transmitting means in the system fortwo-way communication, first and second resistance means adapted to beplaced in series and parallel, respectively, with the receiving meansfor reducing the volume output of said receiver and maintaining thecombined resistance of both said resistances and voice receiving meanssubstantially constant when said switch is operated into its'displacedposition.

8. In an intercommunication system, the combination including aplurality of stations, separate voice receiving and transmitting meansat each station, an amplifier at each station, means at each stationadapted to be placed in circuit with the receiving means for reducingthe volume obtainable therefrom, interstation conductors betweenstations, and switch means at each station for controlling theestablishment of communication circuits between stations, said switchmeans being so constructed and arranged with respect to the apparatusand circuits that they normally condition the receiving means at thevarious stations for reception at high volume of calls from theremaining stations and are manually operable to place said volumereducing means in circuit with the receiving means to establish twowaycommunication between stations at reduced volume.

9. In an intercommunication system, the combination including aplurality of stations, sepaeach station, inter-station conductorsbetween" stations, and switch means at each station constructed andarranged normally to connect the voice receiving means thereat to aninter-station conductor for reception oi a call from another station andshort-circuit the amplifier output, and operable to a displaced positionsimultaneouslyto render said amplifier operative and to connect theamplifier output toan inter-station conductor.

l0. In an intercommunication system, the combination including aplurality of stations, separate voice receiving and transmitting meansat each station, 4normally inoperative amplifier means conditioned forimmediate operation at each station, interstation conductors betweenstations, switch means at each station constructed and arranged normallyto connect the voice receiving means thereat to an inter-stationconductor for reception of a call from another station and short-circuitthe amplifier output, and operable to a displaced positionsimultaneously to render said amplifier operative and to connect theamplifier output ,to an inter-station conductor, and means biasing saidswitch means for movement from said displaced to said normal position.

11. In an intercommunication system, the combination including aplurality of stations, separate voice receiving and transmitting meansat each station, an ampliiier at each station, inter-station conductorsbetween stations, andswitch means at each station for controlling theestablishment of communication'circuits between stations, said switchmeans being so constructed and arranged with respect to tlie apparatusand circuits that they normally condition the receiving means at thevarious stations for reception of calls from the remaining stations athigh volurne and are operable to displaced positions in which two-Waycommunication between stations can be maintained at reducedvolume.

12. In an intercommunication system, the combination including aplurality of stations, voice receiving and transmitting means at eachstation, an amplifier at eachv station, inter-station conductors betweensaid stations, and a single communication control switch at each stationfor controlling communication between stations, each of said controlswitches being so constructed andl arranged with respect to theapparatus that it is operable to a first position to condition thestations for reception through amplifiers at remote stations, toa secondposition, from which it is automatically returned to said first positionupon release, to condition the station for transmission through its ownamplifier, and to a third position in which it is adapted to remainuntil manually moved therefrom to condition the station for `bothtransmission and reception, whereby two-way communication canbemaintained between stations without manipulation of switches afteroperation oisaid switches at two stations to their third positions.

13. In an intercommunication system, the combination including aplurality of stations, voice receiving and transmitting means at eachstation, amplifier means at each station, and plural position switchmeans at each station for controlling the establishment of communicationcircuits between said stations, said switch means being so 'constructedand arranged with respect to the apparatus and circuits that they areopers tween' Vsaid stations, said switch means being so constructed andarranged with respect to the apparatus and circuits that they areoperable to positions in which they condition the stations for receptionat high volume, are operable to l other positions to condition thestations for transmission at high Volume and to further positions tocondition the stations for reception at'reduced volume and transmissionat high volume, and means biasing said switches for movement from saidother positions to said normal positions.

15. An intercommunication system, including in combination a pluralityoi stations, independent inter-station conductors for each station,separate voice receiving means and transmitting means at each station,an amplifier at each station conditioned for immediate operation uponconnection of a B supply thereto, the transmitting means at each stationbeing connected directly to the input of the amplifier thereat, meansincluding separate plural position switches, all mechanicallyinterconnected for simultaneous operation for controlling theconnection' of the output of theamplifler to inter-stationl conductorsother than that associated with the station, the connection of thereceiving means to the inter-station conductor associated with thestation, the connection of the "B supply to the amplifier, and theconnection of a first resistance in series and a second resistance inparallel to the receiving means, said switches being operable to anormal position in which the receiving means is connected directly toits associated inter-station conductor, the amplier B supply isdisconnected, the output thereof is short-circuited, whereby thestations are all conditioned for reception from the other stations athigh volume, said switches being operable to a displaced position inwhich they remain until manually operated therefrom and in which thereceiving means is connectedto its associated inter-station conductorthrough said series resistance, the amplifier is connected to the Bsupply, the second resistance is connected in parallel to the receivingmeans, and the output ci the amplifier is connected to the otherinter-station conductors, whereby two-way communication may bemaintained betweenany two stations at reduced volume without necessityof further manipulation of switches.

16. An intercommunication system, including in combination a pluralityof stations, independent inter-station conductors for each station,separate voice receiving means and transmitting means at each station,an amplifier at each station conditioned for immediate operation uponconnection of a B supply thereto, the transmitting means at each stationbeing connected directly to the input of the amplier thereat, meansincluding separate plural position switches, all mechanicallyinterconnected lfox simultaneous operation for controlling theconnection d the output of the amplifier to inter-station oductors otherthan that associated with the station, the connection oi' the receivingmeans to the inter-station conductor associated with.

' the remaining inter-station conductors, whereby the stations are allconditioned for reception from the other stations at high volume, saidswitches being operable to a displaced position in which they areadapted to remain until manually operated therefrom and in which thereceiving means is connected to its associated inter-station conductorthrough said first resistance, the amplifier is connected to its "Bsupply, the second resistance is connected in parallel to the receivingmeans, and the output of the ampliiier is connected to the otherinterstation conductors, whereby two-way communication may beestablished between any two stations at reduced volume and withoutfurther manipulation oi' switches, and said switches being operable toanother displaced position in which the receiving means is connecteddirectly to its associated inter-station conductor, the amplifier isconnected to its B supply, and the output of the amplier is connected tothe other interstation conductors, whereby a station may be conditionedfor transmission and reception at high volume without furthermanipulation of switches thereat.

1'1. An intercommunication system, including in combination a pluralityof stations, independent inter-station conductors for each station,separate voice receiving means and transmitting means at each station,an ampli-iler at each station conditioned for immediate operation uponconnection of a B supply thereto, the transmitting means at each stationbeing connected directly to the input of the amplifier thereat, meansincluding separate plural position switches, all mechanicallyinterconnected for simultaneous operation for controlling the connectionof the output of the amplifier to inter-station conductors other thanthat associated with the station, the connection of the receiving meansto the inter-station conductor associated with the station, theconnection of the B supply to the amplier, and the connection of a`first resistance in series and a second resistance in parallel to thereceiving means, said switches being operable to a normal position inwhich the receiving means is connected directly to its associatedinter-station conductor, the ampliner "B supply is disconnected, theoutput thereof is short-circuited and disconnected from the remaininginter-station conductorsf whereby the stations are all conditioned forreception from the other stations at high volume, said switches beingoperable to a displaced position f in which they are adapted to remainuntil manually operated therefrom and in which the receiving` means isconnected to its associated inter-station conductor through said iirstresistance, the amplifier is connected to its B supply, the secondresistance is connected in parallel to the receiving means, and theoutput of the amplifier is connected to the other interstationconductors, whereby two-way communication may be established between anytwo 4stations at reduced volume and without further manipulation ofswitches, said switches being operable to another displaced position inwhich the receiving means is connected directly to its associatedinter-station conductor, the amplifier is connected to its B" supply,and the output of the amplifier is connected to the other interstationconductors, whereby a station may be conditioned for transmission andreception at high volume without further manipulation of switchesthereat, and said switches being operable to a further displacedposition from which it is biased to return to its normal position inwhich the receiving means is disconnected from its associatedinter-station conductor, the amplifier is connected to its B supply andthe output of the amplifier is connected to the interstation conductors,whereby any station can call any other station.

18. An intercommunication system, including in combination a pluralityof stations, independent inter-station conductors for each station,separate voice receiving means and transmitting means at each station,an amplifier at each station conditioned for immediate operation uponconnection of a B supply thereto, the transmitting means at each stationbeing connected directly to the input of the amplifier thereat, meansincluding separate plural position switches, all mechanically inter-connected for simultaneous operation for controlling the connection ofthe output of the amplifier to interstation conductors other than thatassociated with the station, the connection of the receiving means tothe inter-station conductor associated with the station, the connectionof the B supply to the amplifier, and the connection of a firstresistance in series and a second resistance in parallel to thereceiving means, said switches being operable to a normal position inwhich the receiving means is connected directly to its associatedinter-station conductor, the amplifier "B supply is disconnected, theoutput thereof is short-circuited and disconnected from the remaininginter-station conductors, whereby the stations are all conditioned forreception from the other stations at high volume, said switches beingoperable to a displaced position in which they are adapted to remainuntil manually operated therefrom and in which the receiving means isconnected to its associated inter-station conductor through said firstresistance, the amplifier is connected to its B" supply, the secondresistance is connected in parallel to the receiving means, and theoutput of the amplifier is connected to the other inter-stationconductors, whereby two-way communication may be established between anytwo stations at reduced volume and without further manipulation ofswitches, said switches being operable to another displaced position inwhich the receiving means is connected directly to its associatedinter-station conductor, the amplifier is connected to its B supply, andthe output of the amplifier is connected to the other inter-stationconductors, whereby a station may be conditioned for transmission andreception at high volume without further manipulation of switchesthereat, said switches being operable to a further displaced positionfrom which it is biased to return to-its normal position in which thereceiving means is and another station can be placed in communicationtherewith without occurrence of feed back by operation of the switchesthereat to their further displaced and normal positions alternately toconnect either the transmitter or receiver to the iirst station.

19. A unit for use in an intercommunicating system, including incombination, a voice reproducer of the loudspeaker type, an interstationconductor, resistance means, and means including a switch and circuitscontrolled thereby for selectively connecting the reproducer directly tosaid interstation conductor or in series with said resistance means tosaid interstation conductor.

20. For use in a multi-station intercommunicating system, thecombination including, a voice reproducer of the loudspeaker type, amicrophone, an amplifier having input and output connections, saidmicrophone being connected to the input connections, a pair ofinterstation circuits, resistance means, and means including electricalcircuits and a switch controlling said circuits operable into a firstposition to connect said reproducer directly to one of said interstationcircuits and to a second position to connect said reproducer to said oneof said interstation circuits through said resistance means and theoutput of said amplier to the other of said pair of interstationcircuits. l

21. In an intercommunicating system, the combination including, aplurality of stations each having a voice reproducer, a microphone, andmeans including a switch and circuits controlled thereby for controllingcommunication between stations, said switch means being so constructedand arranged in said circuits that one-way communication in eitherdirection can be carried on through predetermined alternate `operationof the switches at two stations, and

two-way communication can be carried on without further manipulation ofsaid switches upon operation thereof into another position.

22. In an intercommunicating system, the combination including, aplurality of stations each having a voice reproducer of the loudspeakertype, a microphone, an amplifier, electrical connections for directlyconnecting the microphone to the input of the amplifier, and meansincluding a switch and reproducer and amplier output circuits controlledthereby for controlling communication between stations, said switchmeans being so constructed and' arranged in said circuits that one-wayloudspeaklng communication in either direction can be carried on throughpredetermined alternate operation of the switches at two stations, andtwo-.way loudspeaking communication can be carried on without furthermanipulation of said switches upon operation thereof into anotherposition.

23. In an intercommunicatlng system, the combination includingA aplurality of stations each having a voice reproducer of the loudspeakertype, a microphone, an amplifier, electrical connections' directlyconnecting the microphone to the input of the ampliiier, and meansincluding a plural position switch and reproducer and amplifier circuitscontrolled thereby for controlling communication between stations, saidswitch means being so constructed and arranged that messages can betransmitted in alternate directions 4between a plurality of stations ata relatively high volume by alternate operation of said switches at thestations and two-way communication can be carried on at a lower volumewithout manipulation of said switches.

24. In an intercommunicating system, the combination including aplurality of stations, separate voice receiving and transmitting meansat each station, normally inoperative amplifier means at each stationconditioned for immediate operation upon the supply of B potentialthereto, interstation conductors between stations, said voicetransmitting means being directly connected to the ampliiier input, andswitch means at each station constructed and arranged normally toconnect the voice receiving means thereat to an interstation conductorfor the reception of a call from another station and to disconnect theoutput of the amplifier from the interstation conductors, and operableto a displaced position simultaneously to render said amplifieroperative and to connect the amplifier output to an interstationconductor.

25. In a multiple station intercommunication system, the combinationincluding, an amplifier, a microphone connected to the input thereof, aloudspeaker at each station, volume reducing means, a control switch andcircuits controlled thereby at each station so constructed and arrangedwith respect to the amplifier, loudspeaker and volumereducing means thatmessages may be transmitted from said station to another similar stationin alternate directions at high volume by alternate operation of theswitches at the two stations and two-way communication may be carried onat reduced volume without manipulation of the switches.

26. In an intercommunication system, the combination including, aplurality of stations, each having separate voice transmitting andreceiving means, said receiving means being adapted to be connected totransmitting means at another station through an interst-ationconductor, an amplifier, circuits for connecting said voice transmittingmeans to the input of the amplifier, means including a resistor fordecreasing the volume of said receiving means, and means including aswitch and circuits controlled thereby for connecting the voicereceiving means at one station directly or through said resistor to saidinterstation conductor.

27. In an intercommunication system, the combination including, aplurality of stations, each having separate voice transmitting andreceiving means, an amplifier, circuits for connecting said voicetransmitting means to the input of the amplifier, said receiving meansbeing adapted to be connected to transmitting means at another stationthrough an interstation conductor andhaving an impedance substantiallyequal'to the impedance of the output of the amplier, means fordecreasing the volume of said receiving means, andl means includingcircuits, resistance means and a switch for connecting ,the voicereceiving means at one station directly to said interstation conductoror through said resistance means, said resistance means being soproportioned and arranged that the impedance of the receiving circuitremains substantially constant.

RICHARD H. CAMPBELL.

